In the late nineteenth and early twentieth centuries, American psychologists began addressing problems related to the intellectual capacity of students enrolled in public schools. This paper focuses on the role and influence of psychologists in addressing these problems, specifically the difficulty of classifying students deemed feeble-minded and backward. This paper discusses the diagnostic theory of the clinical psychologist John Edward Wallace Wallin (1876-1969), who was one of America's leading proponents of diagnosing and classifying students in public schools. I show that, in addressing treatment for the feeble-minded and backward, he emphasised educational instruction suited to a child's individual needs and adopted individual/multiphase methods rather than primarily focusing on efficiency-based diagnostic methods. In a democratic society, such as the USA, it was important to establish educational placement and differentiated instruction suited to individual needs.